Crystal type of titanium dioxide (TiO2, titania) includes rutile type and anatase type, and it is known conventionally that anatase-type crystalline titanium dioxide doped with nitrogen develops a photocatalytic function in response to visible light (Nonpatent document 1). The photocatalytic function includes a powerful oxidation action and a superhydrophilic action. For example, the oxidation action is utilized to coat a wall and a floor of an operation room in a hospital with titanium dioxide to provide the sterilization treatment. The superhydrophilic action is utilized to coat a side view mirror of a car and a road mirror with titanium dioxide to provide a glass antifog treatment capable of self-cleaning if it rains, and also is applied to prevent an external wall of a building and a tent sheet from being stained.
Then, a stain on a responsive glass membrane of a pH electrode causes an asymmetric potential which causes an error in the measurement value. In order to maintain an accuracy of the measurement, it is necessary to clean the responsive glass membrane thoroughly using a detergent and the like to remove the stain stuck thereto each time the measurement is performed.    Nonpatent document 1: Kazuhito Hashimoto, et al., Hikari shokubai, Kiso/Zairyo Kaihatsu/Oyo (Photocatalysis, Fundaments/Material Development/Application), issued on Jun. 22, 2004 published by NTS Inc.